Werner's Theory Numerical: Predicting the Complex Formula
One of the foundational concepts in Coordination Compounds is Werner's Theory, which introduced the idea of Primary Valency (ionizable) and Secondary Valency (non-ionizable).
In JEE Main and NEET, this is frequently tested using a Silver Nitrate (AgNO3) precipitation reaction. Let's break down a classic Previous Year Question (PYQ) to see how mole concepts intertwine with coordination chemistry.
Video Tutorial: The Mole Concept Approach
Watch Abhishek Sengar sir from CHEMCA solve this exact JEE Main PYQ using a simple step-by-step ratio method.
Step-by-Step Solution Breakdown
When a complex is dissolved in water, only the ions outside the square bracket (the Primary Valency) will ionize and react. The ligands trapped inside the coordination sphere (the Secondary Valency) will not react.
Therefore, the amount of AgCl precipitate formed tells us exactly how many Chlorine atoms are outside the bracket.
Problem Statement:
On treatment of 100 mL of 0.1 M solution of CoCl3 · 6H2O with excess AgNO3, 1.2 × 1022 ions are precipitated. Find the formula of the complex.
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Calculate the Moles of the Complex:
Moles = Molarity × Volume (in Liters)
Moles = 0.1 M × 0.1 L = 0.01 moles of Complex. -
Calculate the Moles of Precipitated Ions (AgCl):
We use Avogadro's Number (NA ≈ 6 × 1023) to convert the number of ions into moles.
Moles of AgCl = (1.2 × 1022) / (6 × 1023)
= 0.2 × 10-1 = 0.02 moles of AgCl. -
Find the Ratio:
0.01 mole of the complex yields 0.02 moles of AgCl precipitate.
Therefore, 1 mole of the complex will yield 0.02 / 0.01 = 2 moles of AgCl.
This means exactly 2 Chlorine atoms must be ionizable (outside the bracket).
Fig: Only the chloride ions outside the coordination sphere react with Silver Nitrate.
Checking the Options: In a multiple-choice question, you simply scan the options for the one that has exactly Cl2 written outside the square bracket. As Sir pointed out, Option B is the correct answer.
Practice Questions for JEE & NEET
Verify your understanding of Primary vs. Secondary Valency with these conceptual questions.
Question 1: A solution of [Co(NH3)3Cl3] is treated with excess AgNO3. How many moles of AgCl precipitate will be formed per mole of the complex?
Answer: Zero (0 moles).
Reasoning:
Look closely at the formula: [Co(NH3)3Cl3]. All three chlorine atoms are located inside the square bracket. This means they are part of the secondary valency, directly bonded to the Cobalt atom. When dissolved in water, this complex does not ionize at all (it is a non-electrolyte). Since there are no free Cl- ions in solution, no AgCl precipitate will form.
Question 2: What is the electrical conductivity (in terms of total ions produced) of the complex [Co(NH3)6]Cl3?
Answer: It produces 4 total ions (a 1:3 electrolyte).
Reasoning:
- The coordination sphere [Co(NH3)6]3+ remains intact as 1 ion.
- The primary valency consists of Cl3 outside the bracket, which ionizes to form 3 Cl- ions.
- Total ions = 1 + 3 = 4. This high number of ions gives it a relatively high molar conductivity in aqueous solution.
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